At present, the formation of an image using paper as a recording medium is mainly performed by offset printing. The offset printing is suitable for mass production because it can provide high-quality prints at high speed, and the production cost per sheet can be greatly reduced because of the mass production. The reason for it is that once a plate is fabricated by a plate-making step, a great number of reproductions can be made on the basis of this plate, and so the proportion of plate making cost required of individual prints can be lessened. Such offset printing has well matched with the needs of the market to date.
When the provision of prints of various kinds and small lots has advanced with the diversification of information in recent years, however, a problem that the plate cost to the individual prints becomes high, and so the price is comparatively expensive has arisen. Further, a further importance has been given to the immediateness of information in recent years, and a demand for an earlier appointed date of delivery in market that “a person wants to immediately obtain a print” is increasing. Even with this demand, the reduction of the number of sheets printed does not line up with the earlier appointed date of delivery under the circumstances because the current offset printing requires a long time for the so-called lead time from the preparation of a manuscript to plate making and preparation for printing (stabilization of printing machines), while the printing time that is an actual processing time is not as long due to the output rate of about 9,000 sheets per minute. In addition, since vast equipment investment is required, and operations of all steps require great skill, the production base is limited, and it takes a long time from completion of the printing to delivery of a print to a customer.
From such change in market demand, attention has been paid to printing by an ink-jet recording system in recent years. The ink-jet recording system is such a system that an image pattern is recorded on a recording medium by non-contact without using any plate and is suitable for printing of small circulation because of the non-plate system. Due to the background of the age that the establishment of electronic (paperless) information has been advanced, and even image information has been able to be easily processed, the ink-jet recording system has become an expected information-recording system in cooperation with the fact that good prints can be immediately provided without need of highly expert knowledge or large-scaled equipment.
By the way, as a recent trend, it has been required that high-quality images can be recorded by the ink-jet recording system irrespective of the kind of a recording medium. However, the ink-jet recording system is significantly affected by the ink absorbency of a recording medium used, and so it is difficult to record a high-quality image irrespective of the kind of the recording medium. In particular, it is hard to record a high-quality image on recording media (including non-absorbent recording media that do not absorb an ink at all) that are poor in ink-absorbing ability. When recording is conducted on a recording medium poor in ink-absorbing ability, a phenomenon called bleeding in which ink droplets that are impacted adjacently mix with each other and/or a phenomenon called beading in which an ink droplet that has impacted previously attracts an ink droplet that is impacted subsequently causing aggregation of the ink droplets may occur, so that image quality is often deteriorated.
In order to prevent such bleeding and beading, it is effective to lower the flowability of inks used on such a recording medium. In the case of the ink-jet recording system, however, only low-viscosity inks high in flowability can be ejected in order to satisfy the ejection stability of the inks. In other words, the ink-jet recording system is required to have conflicting properties that ink flowability upon ejection must be made high, while ink flowability on a recording medium must be lowered.
In order to satisfy such conflicting requirements, there has been proposed such a system (image forming system using an intermediate transfer medium) that an ink image is formed on an intermediate transfer medium (hereinafter also referred to simply as “transfer medium”), and the ink image formed on the transfer medium is transferred to a desired recording medium to form the ink image on the desired recording medium (see, for example, U.S. Pat. Nos. 4,538,156 and 5,099,256, and Japanese Patent Application Laid-Open No. 62-92849). In this system, an ink ejected from an ink-jet head is caused to impact the intermediate transfer medium to lower the flowability of the ink to some extent, and an image formed by the ink, the flowability of which has been lowered, is then transferred from the transfer medium to a recording medium.
In order to achieve a high quality ink image on the recording medium after the transfer in the image forming system using such an intermediate transfer medium, it is important to obtain a high quality ink image on the intermediate transfer medium before the transfer. For that purpose, it is required to improve the ability (the degree of retaining the ink at the impact position without moving the ink from this position) to hold the ink image on the intermediate transfer medium. In the above-described reference documents, however, the ability to hold the ink image on the intermediate transfer medium is low, and so beading or bleeding occurs on the intermediate transfer medium like the above-described non-absorbent recording medium. It is accordingly desirable to lower the ink flowability on the intermediate transfer medium.
Various proposals have been made to solve such a problem involved in the transfer type ink-jet recording. For example, Japanese Patent Application Laid-Open No. 7-223312 has proposed a method in which an ink is ejected on an intermediate transfer medium as a hot-melt ink by heating an ink-jet head and an ink feed line, and the flowability of the ink is lowered by heat dissipation.
Japanese Patent Application Laid-Open No. 5-330035 has proposed a method in which an intermediate transfer medium is heated to facilitate the evaporation of water in an ink applied to the intermediate transfer medium, thereby lowering the flowability of the ink on the intermediate transfer medium.
Japanese Patent Registration No. 2916864 (JPA 6-240195) has proposed a method in which an intermediate transfer medium is coated with a liquid (reactive liquid) reactive to an ink, and droplets of the ink are caused to impact on this transfer medium to react the ink with the reactive liquid, thereby lowering the flowability of the ink on the intermediate transfer medium.
However, a technique by which the flowability of the ink on the intermediate transfer medium can be lowered to improve the ink image on the intermediate transfer medium, and also the ink image on the recording medium after the transfer, has not been realized by any of the above-described documents.
For example, in the case where the hot-melt ink is used like Japanese Patent Application Laid-Open No. 7-223312, one must use an ink containing a binder in an extremely great proportion to a coloring material for the purpose of developing the phase change property (solid-liquid phase change by heat) of the ink. Therefore, the amount of the ink to be applied increases for the purpose of achieving a desired density. As a result, the thickness of the ink of an outputted image becomes great, thus leading to deterioration of image quality. In other words, this system cannot improve the quality of the ink image on the recording medium after the transfer, because a sense of incompatibility occurs in the ink image on the recording medium after the transfer due to the great thickness of the ink applied. In addition, since the ink solid at ordinary temperature is used, the ink within an ink flow path must be heated and melted upon start-up, and so it takes time to output an image. Further, since there is need to retain this state during operation, vast energy is required.
Beading and/or bleeding cannot be prevented by simply heating the transfer medium like Japanese Patent Application Laid-Open No. 5-330035 because the beading or bleeding of inks occurs before the ink flowability is sufficiently lowered, since the beading or bleeding occurs in the moment at the impact of the inks. This system only achieves the effect of drying the ink image formed on the transfer medium. The ability to hold the ink image on the transfer medium is still low, and so the quality of the ink image on the transfer medium cannot be improved, and the quality of the ink image on the recording medium cannot also be improved.
According to Japanese Patent Registration No. 2916864 (JPA 6-240195), the flowability of the ink on the intermediate transfer medium can be lowered. However, the quality of the ink image on the intermediate transfer medium cannot be made high. More specifically, since the reactive liquid itself is liquid, the beading of the reactive liquid occurs on the intermediate transfer medium when the intermediate transfer medium is coated with such a reactive liquid. As a result, an ink is ejected in such a state that the beading of the reactive liquid has occurred, so that the ink cannot be held at a normal position to fail to make the quality of the ink image on the transfer image high. Incidentally, when an intermediate transfer medium having a surface excellent in ink absorbency is used, a high-quality ink image can be formed on the intermediate transfer medium. However, the ink image sticks to the intermediate transfer medium, so that the ink image cannot be successfully transferred to the recording medium, and moreover cleaning becomes difficult.
As apparent from the above, to form a high-quality ink image on various recording media including recording media (for example, non-absorbent recording media) that are poor in ink-absorbing ability has not yet been realized, without being affected by the ink absorbency of the recording media, even in methods using the intermediate transfer medium. The same applies to a system in which an ink is directly ejected on a recording medium without using any intermediate transfer medium.